How to organize a Project to obtain sub-totals of validated work

is it the right way ?

Yes, It is a good way

What's the impact of activity type on the behavior of the application ?

Some fileds may be defined as mandatory depending on type. Also, workflow can be different.
Have a llok at Type fields.

why didn't I get, in my phase, the amount of "validated work" of the sub-activities ?

Because it is designer this wa : in most cases, you don't know the validated work for sub-activities but for main ones, or just for phases.
So your entry should not be erase by null vlues of sub-activities.
An improvement will come in V4.4 to define how you'd like to sum-up validated work .